ABSTRACT Human pluripotent stem cells (PSCs), particularly human induced PSCs (iPSCs) from patients, offer a novel model system to reveal cellular and molecular events underlying normal and abnormal development, as well as a means to better understand human disease pathogenesis. Studies using iPSCs are especially valuable for enhancing our understanding of complex traits associated with Down syndrome (DS) that cannot be fully recapitulated in animal models. Yet, despite their incredible utility, human PSC studies encounter issues that significantly impede research progress in the field. Variability in iPSCs hinders the progress to identify robust phenotypes and limits utility for drug screening. To address this challenge for Down syndrome, mosaicism (the presence of cells that are derived from a single zygote and are genetically identical except for the presence [or absence] of an additional chromosome 21) can be exploited to generate isogenic cells. We aim to work with individuals who have mosaic DS to create isogenic trisomy 21 and euploid iPSCs to validate disease related phenotypes.